The present invention relates to the processing of semiconductor wafers and, more particularly, to a batch process for forming metal plugs in dielectric layers of semiconductor wafers.
The prior art process, of which the present invention is an improvement, is described in "A 3-Level, 0.35 .mu.m interconnection process using an innovative, high pressure aluminum plug technology" (Z. Shterenfeld-Lavie, I. Rabinovich, J. Levy, A. Haim, C. Dobson, K. Buchanan, P. Rich and D. J. Thomas, Twelfth International VLSI Multilevel Interconnection Conference, Jun. 27-29, 1995, Session II Paper C), which is incorporated by reference for all purposes as if fully set forth herein. Briefly, holes for contacts and vias are etched in a dielectric layer on a semiconductor wafer; a layer of soft metal, typically aluminum, at least as thick as the holes are wide is deposited above the holes, typically by sputtering; and external isostatic pressure is applied to the wafer to force the metal into the holes. According to the prior art process, all of these steps are performed in vacuum chamber cluster machines such as the one shown schematically in FIG. 1.
The particular configuration of a vacuum chamber cluster machine 10 shown in FIG. 1 is used for the deposition and external pressure steps. Cluster machine 10 includes a central hexagonal dealer 12 to which are attached three vacuum chambers 14, 14' and 14", a high pressure chamber 16, and two vacuum cassette handlers 18. The machine is controlled by a control system 20. Vacuum cassettes holding semiconductor wafers are introduced to cluster machine 10 and removed from cluster machine 10 via vacuum cassette handlers 18, and moved counterclockwise from chamber to chamber for processing by dealer 12. In vacuum chamber 14, the wafer is preheated to 450.degree. C. to outgas it, and a soft sputter etch precleaning is performed to clean the surface of the dielectric layer. In vacuum chamber 14', a 200 .ANG. layer of titanium and a 500 .ANG. layer of titanium nitride are deposited. In vacuum chamber 14", the aluminum layer is deposited. In high pressure chamber 16, the wafer is heated to about 430.degree. C. and an inert gas such as argon, under a pressure of 700 atmospheres, forces the aluminum into the holes previously etched in the dielectric layer below. Finally, the wafer is returned to vacuum chamber 14' and a 450 .ANG. layer of titanium nitride is deposited as an anti-reflective coating.
This prior art process suffers from certain inefficiencies associated with the need to operate under both vacuum and high pressure conditions within the same machine. The cluster machine of FIG. 1 processes one wafer at a time, and most of the time is spent raising and lowering the pressure of chamber 16, not actually processing the wafer. In addition, the design and construction of cluster machine 10 is significantly more complex than the design and construction of cluster machines that do not mix vacuum chambers and high pressure chambers, reflecting the special requirements associated with safely cycling between vacuum and high pressure every few minutes.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method, of forming metal plugs in dielectric layers of semiconductor wafers by external pressure, that is more efficient and safer than the presently known methods.